1. Field of the Disclosure
This invention relates to an optical system and, in particular, to an optical assembly configured to monitor and measure the power of randomly polarized light.
2. Prior Art Discussion
A light signal propagating along a powerful laser system may vary within a broad range. The instability of the propagating signal detrimentally affects the task to be performed by the laser system and the functionality of the system's components. To monitor the variation of power of light signals, optical laser systems are provided with power monitoring assemblies discussed below.
FIG. 1 illustrates a power monitoring assembly 10 including a pigtailed linearly-polarized isolator 12 configured to support propagation of a light signal Ii between input and output pigtailed fibers 14 and 16, respectively as disclosed in U.S. patent application Ser. No. 12/072,597 commonly owned with the present application and fully incorporated herein by reference. The isolator core 18 is provided with a tap coupler monitor 20 having a plate-shaped beam splitter 22 operative to bleed off a small portion of optical signal which is coupled into a photo-detector 24, 26 for further measurements. The assembly 10 has been successfully used for power readings of a linearly-polarized light.
However, many known optical applications, including fiber laser systems, operate with randomly polarized light which may affect power readings of monitor 20 for the following reasons. When light travels through matter it suffers power loss. One of the contributors to power loss is polarization. As an optical signal passes through at least partly transparent material, the signal's optical power reduces in selective directions due to spatial polarization interaction. In other words, the energy of the light is divided between two polarization states, “p” and “s”, which are orthogonal with respect to one another. The state of polarization refers to the distribution of light energy between these two modes. The difference in the loss between the two polarization modes represents the polarization dependent loss (PDL) of the device.
Based on the above, fiber laser systems with randomly-polarized light may, thus, be characterized by different coefficients of reflection (Rp, Rs) of for respective “p” and “s” polarization states of light incident, for example, on plate-shaped beam splitter 22 of assembly 20. As a consequence, the power of the tapped off beam fluctuates depending upon the polarization of the incident light.
The efforts directed to provide plate-shaped beam splitter 22 with a coating, which may remedy the effect of the randomly polarized light by having Rs and Rp match one another with the desired degree of precision, were not successful. The latter can be explained by technological limitations of current devices monitoring light within a range in which a coefficient of reflection R does not exceed a fraction of 1%. Only when the coefficient of reflection R is about 20%, the above relationship between Rp and Rs may be consistently satisfied. But 20% would constitute an unacceptably high loss of power.
FIG. 2 illustrates an alternative configuration of power monitoring assembly. The assembly is configured as a fiber tap or fiber coupler 28. A portion of light guided by a fiber 30 is coupled into a fiber 32 which delivers it to a photo-detector 34. Such a power monitoring assembly is effective at low powers not exceeding about 10 W.
FIG. 3 illustrates still a further configuration of a power monitoring assembly 36 configured to detect power scattered along a length of waveguide 40 (Rayleigh scattering). The scattered light in the core of waveguide 40 is detected by a photodiode 42. However, Rayleigh signals may be rather weak and, thus, require sophisticated detectors. Also, it may be difficult to separate direct and backreflected signals from one another.
A need, therefore, exist for an optical unit receiving a fraction of randomly polarized signal light and capable of optically treating the fraction so that the power of the beam, exiting the unit, is polarization independent.